1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device comprising a high concentration diffused region.
2. Description of Related Art
As a nonvolatile semiconductor memory device constituting such memory cells as ROM (Read Only Memory), a device with a configuration including MOSFET (Metal Oxide Semiconductor Field Effect Transistor), which is comprised of a normal gate electrode (generally called a control gate), and a floating gate, which is electrically insulated from its surroundings, has been known.
Such a nonvolatile semiconductor memory device has an active region in the memory cell region thereof. In the memory device, a floating gate (electrode) and a control gate are formed so as to overlap via an insulation film, in a direction perpendicular to the channel direction of a channel region, which is created between the source region and the drain region, crossing the active region. In an ordinary state, the floating gate is formed overlapping with the active region, so the nonvolatile memory device is read normally.
However if the active region and the floating gate are misaligned in the channel direction for any reason during manufacture, a part of the active region which does not overlap with the floating gate, that is a non-overlapping region, exists.
If such an non-overlapping region exists, the data of the memory cells, including the nonvolatile semiconductor memory device, may not be read, and the normal operation of the nonvolatile semiconductor memory device cannot be expected.
It is an object of the present invention to provide a nonvolatile semiconductor memory device which operates normally, even if there is a region where the floating gate does not overlap in the active region, and a manufacturing method thereof.
To achieve the above mentioned objects, the nonvolatile semiconductor memory device of this invention comprises a first conductivity-type semiconductor substrate where an active region is created, a floating gate which is formed on the first conductivity-type semiconductor substrate, and a control gate which is formed on the floating gate. This nonvolatile semiconductor memory device has a first conductivity-type high concentration diffused region in a non-overlapping region with the floating gate in the active region.
Just like the above mentioned example, when data on the nonvolatile semiconductor memory device of this invention is read, the threshold voltage is high when the nonvolatile semiconductor memory device is in the xe2x80x9c0xe2x80x9d state, and the threshold voltage is low when the nonvolatile semiconductor memory device is in the xe2x80x9c1xe2x80x9d state. In other words, according to the nonvolatile semiconductor memory device of the invention, the threshold voltage in the first conductivity-type high concentration diffused region is higher than the threshold voltage of the region outside the first conductivity-type high concentration diffused region in xe2x80x9c0xe2x80x9d state. Therefore when data is read, the threshold voltage of the nonvolatile semiconductor memory device is determined by the threshold voltage in a region other than the non-overlapping region, regardless the threshold voltage in the first conductivity-type high concentration diffused region of the nonvolatile semiconductor memory device, that is the non-overlapping region of the floating gate. As a result, the nonvolatile semiconductor memory device of this invention operates normally even when a non-overlapping region of the floating gate exists in the active region due to misalignment during patterning of a floating gate, for example.